1. Field of the Invention
The present invention relates first of all to an arrangement and to a method for adjustment of the slope times for one or more drivers essentially independently of external conditions. The invention also relates to a driver circuit and to advantageous applications.
2. Description of the Related Prior Art
Drivers and driver circuits are known, for example, as pad drivers for integrated circuits such as microcontrollers, microprocessors, ASICs, memory modules or the like and, to a major extent, govern the electromagnetic compatibility (EMC) of digital assemblies, for example of controller devices for motor vehicle or automation technology.
The relatively large current transients (dI/dt) which are required in order to charge and to discharge the generally capacitive loads, which are connected to the drivers, in a relatively short time to a specific voltage potential are a critical factor for such drivers and driver circuits. These times, which are referred to as the rise times (trise) and fall times (tfall) relate to the respectively rising and falling slope and are quoted for a specific maximum load in the specification for the driver. Normally, the driver strength is designed such that the driver comprises the guaranteed driver and timing characteristics in the worst permissible environmental conditions (referred to as the worst case).
For CMOS circuits, which are known per se, such worst case conditions are, for example, high ambient temperatures, low operating voltages, poor production parameters, that is to say production parameters which lead to xe2x80x9cslowxe2x80x9d switching times, and a maximum load, or the like. Since worst case conditions generally occur only rarely, the output slopes of conventional drivers designed in such a way are very much steeper than necessary, thus making the disturbance spectrum worse. The slope timesxe2x80x94rise time/fall timexe2x80x94are likewise not constant either, owing to the possibility of the environmental conditions changing.
An ideal driver in terms of electromagnetic compatibility has the maximum permissible slope times, which are independent of external conditions, such as the ambient temperature, the operating voltage, the individual production parameters, and the connected load or the like. Furthermore, the maximum permissible slope times are dependent on the respective application of the drivers. For example, memory buses require a much shorter access time than, for example, input signals for an electrical switch (referred to as a smart power switch).
Experiments have shown that a significant improvement in the electromagnetic compatibility of output drivers can be achieved by reducing the driver strength and, associated with this, by increasing the slope times.
There is thus a need to be able to adjust the slope times of drivers individually.
DE-A-44 415 23 C1 discloses a digital driver circuit for an integrated circuit, in which the driver circuit can be matched to a specific application by the operator himself. Depending on the nature of the application, in this case depending on the load capacitance of a component which is to be operated using the driver circuit, this operator must enter an appropriate figure in an input apparatus. The figure is a variable which is dependent on the application. Physically, this circuit is a controller. The known solution assumes that variations in the environmental conditions can be established unambiguously by measuring the saturation current of a measurement transistor.
EP-A-0 436 316 A1 also specifies a circuit arrangement, which matches the impedance of a driver to a connected network. In this case, the circuit arrangement is intended to be suitable for being able to match itself to a load with an initially unknown impedance. To do this, the circuit arrangement has a driver with a predetermined impedance. An element for optional selection of an impedance is connected to the driver and, on operation, selectively causes changes to the predetermined impedance to match a desired, different impedance of the connected load. A further element is connected to this element for selection of the impedance. This element measures the impedance difference between the output of the driver and a digital circuit which is connected to it. A suitable impedance is selected on the basis of the measured values from the element for selection of the impedance, and is added to the predetermined, impedance, that is to say the preset impedance of the driver.
One object of the present invention is to provide an improved arrangement, and a method for adjustment of slope times for one or more drivers, as well as an improved driver circuit, by means of which, in particular, the slope times for one or more drivers can be adjusted in a simple manner, and essentially independently of external conditions.
According to the first aspect of the invention, this object is achieved by an arrangement for adjustment of the slope times for one or more drivers essentially independently of external conditions, having an apparatus for recording the time profile of an output voltage which is emitted from the driver or drivers to a load, having an apparatus for conversion of the measured time profile of the output voltage to an output voltage value, having an apparatus for production of a reference voltage value, having a device for presetting a desired slope time, which is essentially independent of external conditions, for the driver or drivers, which device is connected to the apparatus for production of a reference voltage value, and having a device for comparison of the output voltage value with the reference voltage value, in which case the comparison device is or can be connected to the driver or drivers.
The arrangement according to the invention makes it possible to adjust the slope times for drivers independently of external conditions. In particular, it is possible to adjust the slope times for the drivers independently of the temperature, operating voltage, production process and load. These external conditions are enumerated purely way of example, so that the slope times can also be adjusted independently of other external conditions which are not explicitly mentioned in this enumeration.
The arrangement according to the invention means that one or more drivers is or are calibrated by a control cycle, which can preferably be repeated, such that the user of such a driver can adjust and set desired slope times, which can be selected within wide limits, largely independently of the external conditions.
The fundamental principle of the arrangement according to the invention is to generate a reference voltage value which corresponds to the desired slope time which can be programmed by the user. This reference voltage value is compared with an actually measured output voltage value. The driver or drivers is or are matched as a function of this comparison result.
When the driver or drivers is or are activated, the driver emits an output voltage. This output voltage is passed to a load. If the load which is connected to the driver or drivers is a load capacitance, this load capacitance is charged to an operating voltage Vdd on the basis of the output voltage which is emitted from the driver or drivers. When the driver or drivers is or are activated, the output voltage at the driver output will rise until the operating voltage Vdd is reached. An analogous situation applies to the converse case.
This time profile of the output voltage is recorded by the apparatus for recording the time profile, and this apparatus is specified in more detail further on in the description.
Since, in general, time values can be compared with one another only poorly, the measured time values are converted to an output voltage value in the apparatus for conversion of the measured time profile of the output voltage. This output voltage value can be temporarily stored.
At the same time, an appropriate reference voltage value is produced in the apparatus for production of a reference voltage value, which will be explained in more detail further on in the description. The reference voltage value is produced in such a way that a desired, freely variable slope time, which is essentially independent of external conditions, is preset for the driver or drivers via a device which allows the user of the arrangement to preset a slope time. For the reasons mentioned above, this time signal is in turn converted to a voltage value, the reference voltage value. The reference voltage value can in turn be temporarily stored.
The output voltage value and the reference voltage value are then compared with one another in the comparison device. This comparison device is likewise connected, or can be connected, to the driver or drivers, so that the driver or drivers, or its or their driver strength, can be matched by means of the signals which are emitted from the comparison device.
Preferred embodiments of the arrangement according to the invention can be found in the dependent claims.
The arrangement according to the invention will be explained in the following text with reference to an example, although, of course, the invention is not restricted to the specific example form. This specific example form serves only to illustrate the relationships in the arrangement according to the invention.
This example assumes that, at the moment at which a driver input changes its state, the driver is switched such that a connected load, for example a load capacitance, is charged or discharged via a driver output line. In the following text, it is assumed that the driver input signal changes from low to high at the time t=0, which means that the previously discharged load capacitance is intended to be charged to the operating voltage Vdd over the further course of time. An analogous situation applies to the converse case.
The apparatus for recording the time profile of the output voltage emitted from the driver or drivers to a load may preferably be in the form of a window comparator.
This window comparator may advantageously have two voltage comparators (CP1, CP2), which are connected to an AND gate.
The time profile of the output voltage at the driver output can be monitored up to the operating voltage Vdd via a window comparator such as this. The output of the window comparator may be switched to high, for example, during the rise time or the fall time of the voltage signal.
The rise time, or the fall time, may, for example, be that time period which a signal requires to change from 10% to 90% of the final voltage.
The two voltage comparators CP1, CP2 may, for example, be designed accordingly. One of the voltage comparators may, for example, produce a corresponding high signal once the voltage signal has exceeded the 10% mark of the final voltage. The other voltage comparator, may, for example, emit a high signal until the voltage signal has reached the 90% mark of the final voltage. The two signals from the respective voltage comparators are joined together in the AND gate. When both voltage comparators produce a high signal, the voltage signal which is emitted from the driver is in its rise time, or in its fall time.
In terms of formulae, the output of the window comparator is thus high when:
fu*Vdd less than Vout less than fo*Vdd, 
where, for example, fu=0.1, fo=0.9, Vdd=operating voltage and Vout=output voltage.
In a further refinement, the apparatus for conversion of the measured time profile of the output voltage to an output voltage value may have a current source, a switch element and a capacitance, in which case the switch element is operated or can be operated via signals from the apparatus for recording the time profile of the output voltage which is emitted from the driver or drivers to a load.
The measured time value is converted to a voltage value in this apparatus. To do this, the switch element may, for example, be closed during the high state of the apparatus in order to record the time profile of the output voltage (advantageously of the window comparator), by which means a previously discharged capacitance (for example a measurement capacitance Cmeas) is charged by means of the current source to an output voltage value (Vmeas).
The apparatus for production of a reference voltage value may advantageously have a current source, a switch element and a reference capacitance, in which case the switch element is operated or can be operated via signals from the device for presetting a desired slope time for the driver or drivers. A structure such as this allows the reference capacitance (Cref) to be charged to a reference voltage value (Vref) in the manner described above.
The period during which the switch element is closed is preset by the user of the arrangement by operating the device for presetting a desired slope time, which is essentially independent of external conditions, for the driver or drivers, which device will be explained in more detail further on in the description. The time period during which the switch element is closed thus corresponds to the desired rise time, or fall time.
In a further refinement, the device for comparison of the output voltage value with the reference voltage value may be in the form of a comparator. This voltage comparator (CP3) compares the output voltage value (Vmeas) with the reference voltage value (Vref). When Vmeas less than Vref, the capacitance for the output voltage has been charged for a shorter time than the reference capacitance. The actual rise time, or fall time, was accordingly shorter than that desired. The voltage comparator (CP3) can indicate this, for example, by outputting a high level, thus reducing the driver capability of a connected driver. An analogous situation applies to the case where Vmeas greater than Vref.
The control cycle starts once again on the next rising or falling slope, so that the driver is successively matched to the load and to the desired slope time.
An apparatus for selection of the driver strength can preferably be provided, with this apparatus being connected to the device for comparison of the output voltage value with the reference voltage value, and in which case the apparatus for selection of the driver strength is or can be connected to the driver or drivers as well. The apparatus can be connected to the driver or drivers via appropriate control lines which are referred to, for example, as a driver enable bus (DEB).
The device for presetting the desired slope time for the driver or drivers can preferably be designed to produce a square-wave pulse, whose length corresponds to the desired slope time.
The essential feature for the quality of the driver regulation, or driver adjustment, is the exact generation of the desired slope time. This must not be dependent on external conditions, such as variations in the production process, fluctuations in the ambient temperature, the magnitude of the supply voltages or the like. On-chip circuits are very largely impractical for this purpose, since they are subject to these influences and compensation would involve major complexity.
It is therefore advantageously possible to provide for the device for presetting the desired slope time for the driver or drivers to be designed for processing a system clock. A system clock such as this is available, for example, in microcontrollers, microprocessors and in most ASICs. The system clock is generally generated from an external crystal, or oscillator, and may be assumed to be constant with regard to the stated external conditions.
If the clock frequency of the system clock is equal to fmc (mc=master clock), then the pulse length t=1/(2*fmc) can be tapped off directly. Different pulse widths of duration t=2{circumflex over ( )}n/(2*fmc) where n greater than 0 can be produced by means of frequency dividers.
If pulse lengths such that t less than 1/(2*fmc) are required, so-called phase locked loop circuits may be used. It is likewise possible to use a circuit which has the features described in the following text.
The device for presetting the desired slope time for the driver or drivers may preferably have one or more delay elements. Each delay element delays the output of the rising slope at its output as a function of a control voltage (Vctrl). The input signal is, for example, a relatively low-frequency system clock which, when a number of delay elements are used, passes through a delay line formed by such delay elements.
Furthermore, the device for presetting the desired slope time for the driver or drivers may have at least one phase detector. A phase detector such as this checks whether the falling slope of the input signal (master clock) occurs at the same time as the rising slope, which is emitted with the delay, at the output from the delay line.
If this is not the case, depending on which level change took place first, the control voltage is changed such that the phase difference becomes less in the next cycle. An apparatus for production of such a control voltage may be provided, for example, in order to produce the control voltage.
The falling slope of the input signal may be passed on virtually without any delay by the delay element, or by the delay elements. If the input signal is in each case linked to the output of each delay element via appropriate AND gates, the input signal itself results in n parallel signals which have high level durations of tmin, 2*tmin, 3*tmin, . . . , n*tmin, where n=the number of delay elements, and tmin=1/(2*n*fmc).
The device for presetting the desired slope time for the driver or drivers may advantageously have at least one switch element, in particular a multiplexer, for switching between different slope signals. This switch element allows the user to select a signal which he desires and which then corresponds to the desired slope time. The switch element is preferably in the form of a multiplexer. Multiplexers such as these are already known per se. They have a decoder which may select a desired one of n inputs, which is then passed to an output. Multiplexers can be produced both with gates and with analog switches using CMOS technology.
A configuration such as this of the device for presetting the desired slope time for the driver or drivers first of all makes it possible to compensate for fluctuations in the external conditions in a manner which is simple but nevertheless very accurate. Furthermore, a device such as this has the advantage that there is a fixed and clear relationship between the selected output pulse duration (slope time) and the system clock frequency.
According to a second aspect of the present invention, a driver circuit is provided for driving a load and has one or more drivers which is or are connected to the load. This driver circuit is, according to the invention, characterized in that the driver or drivers is or are connected to an arrangement according to the invention as described above, for adjustment of the slope times.
The driver circuit according to the invention makes it possible for the driver or drivers to be calibrated by means of anxe2x80x94advantageously repeatingxe2x80x94control cycle in such a manner that the slope times which are desired by the user and which can be selected within wide limits can be set or can be achieved largely independently of external conditions. With regard to the advantages, influences, effects and method of operation of the driver circuit according to the invention, reference is likewise made to the above statements relating to the arrangement according to the invention and to the following statements relating to the method according to the invention, whose entire contents are explicitly referred to here.
Preferred embodiments of the driver circuit can be found in the dependent claims.
The driver or drivers may preferably be in the form of a scalable driver or scalable drivers.
Each driver may advantageously comprise one or more driver elements. The use of a number of driver elements is known per se and is described, for example, in DE-195 45 904.0, which was likewise submitted by this applicant and whose disclosure content is to this extent also included in the description of the present invention.
If a scalable driver comprises a specific number of driver elements, preferably in parallel, these driver elements can be enabled or inhibited individually by means of control lines (driver enable bus DEB). When the driver input changes its state, all the enabled driver elements switch in a corresponding manner and charge, or discharge, a connected load via the driver line output.
The splitting of the driver into a number of partial driver elements has the further advantage that a high output impedance can be achieved at a low driver power level, and that, in consequence, it is possible to reduce the extent to which disturbances are coupled onto the supply lines. Furthermore, it is worthwhile to make the transistor widths in each driver stage each twice as great as those in the next lower stage. This means that it is possible to select the driver stages by means of a driver strength selector in the form of a binary counter, so that the resultant driver strength range can be covered in equal steps.
The increase or decrease in the driver strength after a control cycle may in this case correspond, for example, to the driver strength of the smallest driver stage.
One or more drivers may in each case advantageously be connected to an arrangement for adjustment of the slope times. As has already been described in the introduction, drivers are preferably used in conjunction with integrated circuits. Circuits such as these are, for example, integrated on a chip, on which only a very small amount of space is normally available. In order to save chip surface area, fewer arrangements for adjustment of the slope times are preferably provided than the number of drivers connected to them.
In this case, the individual drivers can each be connected to one arrangement via suitable switch elements, for example a multiplexer as already described above. This makes it possible to successively regulate the strengths of a number of output drivers, which are advantageously scalable, in the process of which the respective, optimum driver strengths can be temporarily stored. Furthermore, a circuit arrangement such as this advantageously makes it possible for the regulation process to be carried out only in certain phases, for example the so-called set-up phases, and for the driver strengths which have been found then to be checked at suitably selected time intervals.
The load which is connected to the driver or drivers may advantageously be in the form of a capacitive load. This is the most frequent configuration form of such loads, particularly in the field of CMOS circuits. Nevertheless, the invention is not restricted to capacitive loads, so that resistive or inductive loads as well as any desired combinations of the individual load types are feasible in all cases.
According to a third aspect of the present invention, a method is provided for adjustment of the slope times for one or more drivers essentially independently of external conditions, which method can be carried out, in particular, using an arrangement according to the invention as described above. This method is characterized by the following steps:
a) measurement of the time profile of an output voltage which is emitted from the driver or drivers to a load;
b) conversion of the measured time profile of the output voltage to an output voltage value;
c) production of a reference voltage value which is essentially independent of external conditions;
d) comparison of the output voltage value with the reference voltage value; and
e) matching of the driver or drivers as a function of the comparison results.
The method according to the invention allows the driver or drivers to be calibrated in a simple manner such that slope times which are desired by the user and can be selected within wide limits can be achieved or set largely independently of external conditions. With regard to the advantages, influences, effects and method of operation of the method according to the invention, reference is likewise made to the above statements relating to the arrangement according to the invention and to the driver circuit according to the invention, whose entire contents are explicitly referred to here.
Advantageous embodiments of the method can be found in the dependent claims.
The fundamental principle of the method according to the invention is to generate a reference voltage value which corresponds to the programmable slope time which is desired by the user. This reference voltage value is compared with an actually measured output voltage value. The driver is matched as a function of this comparison result.
The time profile of the output voltage can advantageously be measured by measuring its rise time and/or its fall time.
The measured time profile of the output voltage can preferably be converted in an apparatus for conversion to an output voltage value. This is advantageous since voltage values can be compared considerably more easily and accurately than would be possible in the case of time values.
The apparatus for conversion of the time profile of the output voltage may advantageously have a current source, a switch element and a capacitance, with the time profile of the output voltage being converted to an output voltage value by closing the switch element during a predetermined time period, in particular during the rise time and/or the fall time of the output voltage, and such that, during this time period, the previously discharged capacitance is charged by means of the current source to the output voltage value.
In a further refinement, the reference voltage value can be produced in an apparatus for production of a reference voltage value, with the apparatus having a current source, a switch element and a reference capacitance, by the switch element being closed via a device for presetting a desired slope time, which is essentially independent of external conditions, for the driver or drivers for the duration of a voltage pulse, in particular a square-wave pulse, which is preset by the device and whose length corresponds to the desired slope time, and, during this time period, with the previously discharged reference capacitance being charged by means of the current source to the reference voltage value.
The output voltage value and the reference voltage value may be compared in a comparator, with the slope time of the output voltages which are emitted to the load being adjusted on the basis of these comparison results.
The individual method steps can be repeated in order to produce a control cycle. In this case, the driver strength of each driver is in each case matched with a delay of one clock cycle. However, this is generally not disadvantageous since the external conditions normally change only slowly.
An arrangement according to the invention and as described above for adjustment of one or more drivers and/or a driver circuit according to the invention as described above and/or a method according to the invention as described above may particularly advantageously be used in conjunction with integrated circuits, with the individual drivers preferably being in the form of pad drivers.